The polymerization of olefins in two or more serially connected gas-phase reactors allows to produce olefin polymers with improved properties and/or to simplify the existing production processes. This is made possible by choosing polymerization conditions in the second reactor or subsequent reactors different from the reaction conditions existing in the first polymerization reactor. Typically, olefin polymers grow on particles including a catalyst component, which continues to exert a catalytic activity even when the polymer particles are transferred to a successive gas-phase reactor. The polymer resulting from the first gas-phase reactor is transferred to the second gas-phase reactor, where polymerization is continued under different conditions. Therefore, different fractions of polymer can grow on the same particle by maintaining a different composition of the gas-phase mixture in each reactor.
Examples of polymers that may be produced by a multistage gas-phase process include bimodal or multimodal polymers obtained by maintaining a different concentration of a chain terminator, such as hydrogen, in each reactor; and random or heterophasic copolymers obtained by polymerizing different (co)monomers in each reactor. The term “heterophasic copolymer” includes also in-reactor polymer blends.
The transfer of the polymer from one gas-phase reactor to another one is a critical step of a multistage polymerization process. A direct discharge of polymer from an upstream reactor to a downstream reactor does not allow maintaining really different polymerization conditions in the downstream reactor, due to the substantial amount of gases and dissolved hydrocarbons associated to the polymer transferred to the downstream reactor.
A solution that has been proposed for a long time is degassing the solid polymer discharged from the upstream reactor, then subjecting the polymer to a compression stage and transferring it to the downstream polymerization reactor. A process according to that solution is disclosed in EP 192 427 A1, which describes a process in which the compression stage is performed by means of the reaction gas mixture of the downstream reactor at a temperature lower by at least 20° C. than the temperature of the downstream reactor. WO 2008/058839 A2 discloses a process for the multistage polymerization of olefins which allows continuously discharging the polymer and the gas reaction mixture from the upstream reactor into a transfer device and continuously feeding polymer from the transfer device to a downstream reactor by using a transfer device comprising a separation chamber, in which the gas reaction mixture is removed from the polymer, and at least a couple of lock hoppers, which work intermittently in parallel.
EP 050 013 A2 refers to a process for polymerizing an olefin in the gaseous phase in a multiplicity of steps in at least two independent polymerization zones connected to each other by a transfer passage by which a gaseous stream containing the polymer obtained in a first polymerization zone is transferred into a second polymerization zone. The process is characterized in that an inert gas zone is provided in the transfer passage and at least a part of the gas components of the gaseous stream containing the polymer is replaced by an inert gas.
These processes have the disadvantage that they comprise a pressure reduction step by which the reaction gas, which was removed with the polymer from the first reactor, is separated from the polymer particles. However, for recycling the reaction gas to the upstream reactor it is needed to compress this reaction gas again. This requires specific equipment and makes the process expensive and energy-intensive.
EP 1 040 868 A2 discloses a method of multistage gas phase polymerization in which a polymerization of a feed gas mixture at least containing ethylene, an alpha-olefin and hydrogen is carried out in an upstream arranged fluidized-bed reactor. The polymer powder taken up from the upstream arranged fluidized-bed reactor is treated with a gas to lower the content of alpha-olefin gas and hydrogen gas in the polymers powder and then introduced into a downstream arranged reactor.
U.S. Pat. No. 7,465,772 B2 describes a method for continuously polymerizing olefin(s) in a plurality of serially-disposed gas-phase polymerization reactors in which an upstream and a downstream reactor being adjacent to each other are connected via a gas exchange vessel containing a gas distributor plate. The polymer powder transferred from the upstream reactor is temporarily accumulated in the gas exchange chamber and the first gas which has been introduced from the upstream reactor together with the polymer powder and which exists in the polymer powder is exchanged at least partly with a second gas which is fed into the gas exchange vessel. The polymer powder is then transferred intermittently from the gas exchange vessel to the downstream reactor.
US 2010/0029867 A1 discloses a gas-phase polymerization apparatus comprising a gas-phase polymerization reactor and a gas separator which is connected to the gas-phase polymerization reactor by a transfer tube. A mixture of a polymer powder and a gas is introduced into the gas separator in which the gas contained in the mixture is replaced by a replacement gas. However, the disclosed set-up requires beside the reactor an additional pressurized vessel, the gas separator, and has the risk that polymer powder gets stuck in the transfer tube.
WO 2007/071527 A1 describes a method of discharging polymer particles from a fluidized-bed reactor in which the polymer is continuously recycled in an outside circulation loop from the gas distribution grid to the upper region of the fluidized-bed reactor and the polymer is withdrawn from the circulation loop. WO 2008/074632 A1 discloses a gas distribution grid of a fluidized-bed reactor having the inlet of the discharge conduit been placed at the center of the distribution grid. The polymer particles discharged through this conduit are fed to the degassing and extruding facilities.
Thus, it was the object of the present invention to find a simple process for transferring polyolefin particles from a first gas-phase polymerization reactor to a second gas-phase polymerization reactor which not only reliably allows to prevent the transfer of the reaction gas mixture of the first gas-phase reactor to the second gas-phase reactor, but which process also facilitates continuous polymerizations in both the first and the second gas-phase polymerization reactor and which does not require a lot of machinery, i.e. can be implemented without high investment costs, and can be carried without the necessity of recompressing or recycling a larger amount of the reaction gas of the first gas-phase polymerization reaction, i.e. can be operated a low operational costs.